With quarter hour rounding enabled, you'd see the following results:
A start time of 9:11 is closer to 9:15 than 9:00 so it rounds to 9:15.
A stop time of 12:10 is closer to 12:15 than 12:00 so it rounds to 12:15.
9:15 to 12:15 is 3:00 hours (rounding to the nearest minute would show 2:59 hours).
A start time of 9:08 is closer to 9:15 than 9:00 so it rounds to 9:15.
A stop time of 12:07 is closer to 12:00 than 12:15 so it rounds to 12:00.
9:15 to 12:00 is 2:45 hours (rounding to the nearest minute would still show 2:59 hours).
You can take a look at what the FLSA says about rounding time on this fact sheet. Since Virtual TimeClock records the actual start and stop times to the exact minute, you can change your rounding rules anytime to see what effect it has on your current timecard reports.